Phase Field Model of Modification of Microstructure of Al-15Mg2Si- 4.5Si Composite by Addition of Sr during Semi-Solid Processing

Semi-solid processing over a cooling slope has emerged as an efficient technique for producing near-spherical grain structures in metallic composites. In this study, an Al-15Mg₂Si-4.5Si composite processed using this method exhibited globular Mg₂Si particles embedded within an α-Al matrix. The addition of 0.01% Sr further refined the microstructure by reducing the size of the Mg₂Si particles, acting as a grain-refining agent. To gain deeper insight into the effect of Sr addition on microstructure evolution during semi-solid processing, a two-dimensional phase field model is developed. Wherein, rather than modelling the nucleation of Sr-containing phases explicitly, the influence of Sr is realised by modifying the relevant phase field parameters. The simulations predicted key microstructural characteristics—including grain size, sphericity, area fraction and grain density of P-Mg₂Si —along with interfacial energy and mobility coefficients for both the base and Sr-modified composites. The phase field results show good agreement with experimental observations, validating the modelling approach. Keywords: Semi-solid, Magnesium silicide, Phase field, grain refinement